As the need for smaller and thinner electronic apparatuses grows in recent years, finer semiconductor devices mounted in these electronic apparatuses have been increasingly demanded, and various proposals have been made for higher exposure resolution to fulfill this demand.
Since a shorter wavelength of an exposure light source is one effective means for higher resolution, recent exposure light sources have shifted from a g-line (with a wavelength of about 436 nm) and an i-line (with a wavelength of about 365 nm) to a KrF excimer laser (with a wavelength of about 248 nm) and an ArF excimer laser (with a wavelength of about 193 nm). In the near future, use of an F2 excimer laser (with a wavelength of approximately 157 nm) is expected to be promising.
A conventional optical element is available to an optical system down to a wavelength region for the i-line, but conventional optical glass cannot be used for such a wavelength region as covers the KrF and ArF excimer lasers and the F2 laser due to its low transmittance. An optical system in an exposure apparatus that uses the excimer laser as a light source has commonly used an optical element made of quartz glass (SiO2) or calcium fluoride (CaF2) having larger transmittance to light with a shortened wavelength, and it has been considered that exposure apparatus that uses the F2 laser as a light source necessarily uses an optical element made of calcium fluoride.
Calcium fluoride single crystal has been manufactured mainly by a crucible descent method or Bridgman method. This method fills highly purified materials of chemical compounds in a crucible, melts in a growth device, and gradually descends the crucible, thereby crystallizing the materials from the bottom of the crucible. The heat history in this growth process remains as a stress in calcium fluoride crystal. Calcium fluoride exhibits birefringence to the stress, and the residual stress deteriorates optical performance. Therefore, a heat treatment follows the crystal growth to remove the stress.
However, calcium fluoride includes a non-negligible amount of intrinsic birefringence that results from its crystal structure even in an ideal crystal that does not have internal stress.
FIG. 7 shows crystal axes in calcium fluoride as a cubic system where axes [1 0 0], [0 1 0] and [0 0 1] are interchangeable. Without influence of the intrinsic birefringence, the optical characteristic is so isotropic that light that propagates in the crystal is affected by variable influence according to its propagation directions.
The intrinsic birefringence of calcium fluoride is described with reference to FIGS. 8 and 9. FIG. 8 indicates a size of birefringence according to ray directions in the crystal. Referring to FIG. 8, the birefringence amount becomes zero to light that propagate along the axes [1 1 1], [1 0 0], [0 1 0] and [0 0 1]. The birefringence amount becomes such a maximum value to light that propagate along the axes [1 0 1], [1 1 0] and [0 1 1] as 12 nm/cm for the wavelength for F2 laser. FIG. 9 indicates a phase advance axis distribution of the birefringence according to the light direction. When an optical system is made of such crystal, a wave front that contributes imaging changes according to polarization directions of incident light, and two split wave fronts approximately form double images. In other words, the intrinsic birefringence would deteriorate the imaging performance of the optical system.
As discussed, while the influence of the intrinsic birefringence varies according to light propagation directions in the crystal, a combination of plural crystals would be able to correct influence of the intrinsic birefringence. When crystal orientations are adjusted so that the light that has been incident upon first crystal in a phase advance direction may enter second crystal in a phase delay axis direction, the light that has passed through two crystals cancels out the advance and delay of the wave front. A conventional optical system in a projection exposure apparatus that uses calcium fluoride have accorded its crystal axis [1 1 1] with the optical axis of the optical system without exception. A method for correcting influence of the intrinsic birefringence has been proposed which accords the crystal axis [1 1 1] with the optical axis of the optical system, and adjusts an angle of the crystal around the optical axis. Another method for correcting influence of the intrinsic birefringence has been already proposed which combines calcium fluoride orienting a crystal axis other than the crystal axis [1 1 1] with calcium fluoride orienting the crystal axis [1 1 1], and adjusts an angle around the optical axis.
However, the amount of the intrinsic birefringence is in reverse proportion to the square of a wavelength and known to be, for example, 3.4 nm/cm for the wavelength of 193 nm of ArF excimer laser (“ArF wavelength”) and 11.2 nm/cm for the wavelength of 157 nm of F2 excimer laser (“F2 wavelength”). For a shorter exposure wavelength, an angular adjustment around the optical axis is insufficient for correction to the crystal axis [1 1 1] of calcium fluoride and desired imaging performance. A glass material orienting a crystal axis other than the crystal axis [1 1 1], e.g., the crystal axis [1 0 0] has a difficulty to reduce the internal stress caused by the manufacture or birefringence resulting from the internal stress, causing deteriorated yield and increased cost.